Common rail injection, also referred to as storage injection, is a fuel injection system for internal combustion engines in which a high pressure pump brings fuel up to a high pressure level. The pressurized fuel then fills a system of pipes which is constantly under pressure during operation of the engine. An essential idea of common rail fuel injection is a complete separation of pressure generation from the actual injection process. This makes an injection controlled exclusively by engine characteristic maps possible. An injection timing and an injection quantity are controlled by an electronic engine control. The engine control controls an electrically actuated fuel injector for each cylinder, the fuel injection valve also being referred to as a fuel injector.
A storage injection system in a four cylinder diesel engine includes, for example, an air flow meter, a control unit, a high pressure pump, a high pressure storage (rail), fuel injectors, a crankshaft speed sensor, a coolant temperature sensor and an accelerator pedal sensor. Such a storage injection system-common rail may vary in the number of fuel injectors depending on the present cylinders. Furthermore, the type of pressure regulation may differ among the various fuel injection systems. For example, in the so-called adjuster concept with radial pumps, the pressure regulator on the rail is dispensed with, or in the case of plug-in pumps, the proportional quantity valve on the intake side of the high pressure pump is dispensed with. In addition, there may be deviations in the type of pre-feed.
When learning a minimum actuation duration of fuel injectors, for example, in accordance with BOSCH's own “zero quantity calibration” concept, starting from a minimal electrical actuation duration which definitely does not result in an injection, the actuation duration during an internal combustion engine coasting, is gradually increased until a change in the engine speed, measurable via a corresponding crankshaft sensor speed signal of the internal combustion engine, occurs as a result of a combusted injected quantity of fuel. Based on a minimum electrical injector-specific actuation duration ascertained in this manner, all injection types, for example pilot injections, are corrected injector-specifically with respect to their actuation duration. An actuation duration is equivalent to the duration of an injection or an injection duration.
In hybrid concepts which allow the internal combustion engine to be shut off, such as for example, during pure electric driving during the parallel hybrid or combustion engine-off coasting, the internal combustion engine is uncoupled and switched off to avoid drag losses. However, this would then fully omit the internal combustion engine coasting phases necessary for the above-described learning method.
In particular for so-called start/stop systems a quick and convenient starting is desired. Factors identified as limiting a starting reliability have included limited controllability of an engine stop into a defined target position, as well as an excessively low combustion moment at high engine temperatures due to excessively low air density. A typical drag moment resulting from charge compression, friction and mass moment of inertia of an internal combustion engine during an engine start amounts to approximately 120 Nm in a gasoline engine, which represents a dynamic gas exchange moment.
Dragging a gasoline engine over its top dead center requires approximately 50 Nm. The essential features that distinguish diesel engines from gasoline engines are the following:
The method of combustion (auto-ignition) and the associated higher compression ratios (cf. charge compression), higher cylinder peak pressures (cf. friction) and the associated heavier weights of the diesel engine with respect to the mass moment of inertia. This results in a greater expenditure during an engine start-up with respect to starting performance and, depending on the number of cylinders, in an irregular stopping of the corresponding diesel power plant compared to the gasoline engine. Due to the comparatively high cylinder peak pressures, approximately double the amplitude of the gas exchange moment from cylinder to cylinder is to be expected in a diesel engine. Associated with that is a greater vibrational excitation of the power plant and an uncomfortable starting and stopping process. To prevent uncomfortable shaking motions of the internal combustion engine due to gas exchange moments, throttle valves in the intake duct and/or exhaust gas flaps of the exhaust gas system of the internal combustion engine are closed. As a result, the cylinder pistons flex against the air columns on the intake and exhaust gas sides.
The dissertation of Dipl.-Ing. Sören Hans-Jürgen Müller, entitled “Der Startvorgang von hybridisierten Ottomotoren” (Start-up process for hybridized gasoline engines) (Darmstadt 2010), in particular Chapter 5, explains the hybrid start-up of a gasoline engine in greater detail.